The present invention relates to a pneumatic tire and a manufacturing method therefor, and more particularly, to techniques related to a body ply which forms a bead portion of a pneumatic tire and the skeleton of the tire.
FIG. 46 shows a cross section of a general pneumatic tire 351 mounted on a vehicle. This tire 351 comprises a tread 351d; a pair of side walls extending inward from both edges of the tread 351d in a radial direction of the tire 351; and a pair of bead portions 351b mounted to a wheel rim 352. A bead 354 made up of a bead core 354a and a bead filler 354b is disposed in each of the bead portions 351b. Both crosswise ends (both axial ends) of a body ply 353 forming the skeleton of the tire 351 are bent to wrap the beads 354 therein.
A radial intermediate portion of the side wall is a maximum cross-sectional width portion 351a at which the tire 351 has a maximum width of cross-section. While the vehicle is running, a large load such as a compression load acts on a portion of the side wall between the maximum cross-sectional width portion 351a and the bead portion 351b, i.e., an inner side portion 351c of the side wall. When edges 353b of bent portions 353a of both crosswise ends of the body ply 353 are arranged in the inner side portion 351c, a stress concentrates on the edges 353b. This concentrated stress causes the occurrence of a crack and growth of the crack on the edges 353b. 
For avoiding such a concentrated stress, the bent portion 353a of the body ply 353 must be extended to the vicinity of the tread 351d beyond the maximum cross-sectional width portion 351a, the edges 353b of the bent portion 353a must be covered with a coating layer, or the bent portion 353a must be reduced in length such that their edges 353b are fixed within a region of a flange of the wheel rim 352. In this event, from the viewpoint of a reduction in the manufacturing cost and weight, it is desirable that the edges 353b of the bent portion 353a be fixed within the region of the flange of the wheel rim 352. However, conventional tire manufacturing methods have failed to meet such requirements as described below.
FIGS. 47(a) to 47(c) show a conventional method for mounting the bead 354 to the body ply 353. In this conventional method, as shown in FIG. 47(a), the body ply 353 formed in a cylindrical shape is first set on a drum 355. In this event, both width edges of the body ply 353 protrude from the drum 355 and are supported by a flat bladder 356. Also, the bead 354 is assembled in a portion of the body ply 353 protruding from the drum 355. It should be noted that FIGS. 47(a) to 47(c) show one crosswise end portion only of body ply 353.
Next, as shown in FIG. 47(b), air is injected into the bladder 356. This causes the bladder 356 to inflate to erect the crosswise end portions of the body ply 353.
Further, as shown in FIG. 47(c), the bladder 356 is continuously inflated, and a push can 357 made of a metal presses the bladder 356 toward the drum 355. This pressing bends the body ply 353 such that a crosswise end portion thereof is bent over, and the bead 354 is wrapped around by the bent portion 353a. Subsequently, the drum 355 is contracted in the radial direction, and the body ply 353 wrapping around the bead 354 is removed from the drum 355 and sent to the next process.
As shown in FIG. 47(b), in an initial stage of the inflation of the bladder 356, a relatively small outer diameter of the bladder 356 fails to sufficiently erect the crosswise end of the body ply 353. For this reason, when the bent portion 353a of the body ply 353 is short, the bent portion 353 hangs down by its own weight, or the bent portion 353a is wrinkled. Therefore, for avoiding the aforementioned problem of concentrated stress, conventional methods must extend the bent portion 353a to the vicinity of the tread 351d, or cover the edge 353b of the bent portion 353a with a coating layer, which however is insufficient to reduce the manufacturing cost and weight.
Also, sport-oriented vehicles which are required to have high running performance often employ tires having a high oblateness. Such a flat tire is required to have a highly rigid and flexible side wall. Such a flat tire must also prevent cords within a body ply from being cut by an impact which could be applied thereto when the tire runs on a cat's eye embedded in a road. To meet such requirements, there exists a flat tire which comprises two layers of body plies 302, 303 as shown in FIG. 48.
The body plies 302, 303 are generally manufactured in the following manner. Specifically, a wide and long tyre fabric cord array comprised of a large number (for example, approximately 1500) of parallel cords 304 is coated with a rubber 305 by a calendar. This results in the formation of a wide and long sheet-shaped product having the cord array embedded in the rubber 305. Next, the sheet-shaped product is cut by a cutter into a required width to fabricate two body plies 302, 303 which are different in width and length from each other. Then, these body plies 303, 303 are sequentially wrapped around a cylindrical drum and overlapped one around the other to form a two-layer body ply.
If both body plies 302, 303 have edges (corresponding to the edge 353b of the body ply 353 in FIG. 46) aligned with each other, a stress will concentrate on the edge portions, so that a crack is more likely to grow from the edges. For this reason, the body plies 302, 303 are made different in width such that the edges of the body plies 302, 303 are not aligned with each other.
A tire disclosed in Japanese Unexamined Patent Publication No. 2001-130215employs a body ply made up of a plurality of layers of cord arrays for improving the strength of the tire for use in large vehicles. The body ply disclosed in this official gazette is manufactured in the following manner. Specifically, tire fabric cord arrays each drawn out of a plurality of reels are stacked, and are coated with a rubber by a calendar. This results in the formation of a wide and long sheet-shaped product having a plurality of layers of cord arrays embedded in the rubber. The resulting sheet-shaped product is used as a material for the body ply.
For manufacturing the wide and long sheet-shaped product for use as a material for a body ply, a large number of reels are installed in accordance with the number of cords, or a large reel must be provided for wrapping a tyre fabric cord array made up of a large number of cords. Furthermore, a large calendar and a large cutter must be installed in accordance with the width of the sheet-shaped product. In addition, a large space must be ensured for transporting and storing the wide and long sheet-shaped product. Thus, the cost of installation required for manufacturing the body ply is increased to cause a higher tire manufacturing cost.
As a technique for solving the problems mentioned above, Japanese Unexamined Patent Publication No. 2001-145961 discloses a technique of manufacturing a sheet-shaped product for use as a material for a complete body ply by wrapping a drum with an elongated ribbon embedded with cords to form a cylinder and cutting the cylinder into the sheet-shaped product. However, for manufacturing two layers of body plies, it is necessary to provide two types of drums each having different diameters in accordance with the sizes of the two body plies. This results in complicated facilities required for manufacturing the body plies, and complicated manufacturing processes. Also, tedious changes in facilities will be involved in a change in the type of tire to be manufactured, which constitutes a factor of a higher tire manufacturing cost.